Semiconductor wafer substrates have been thinned by wet etch for a number of reasons for many years. Post grind stress relief (to remove grind marks and surface damage) is an example. The wet process is a chemical reaction. One chemistry commonly used for isotropic wet etching of silicon is a combination of nitric acid and hydrofluoric acid (HF), along with other diluting agents not involved in the stoichiometry of the reaction. The nitric acid acts as an oxidizer to convert the surface into silicon dioxide and then the HF etches (dissolves) the oxide. The reaction proceeds as shown below and has been well documented in the literature.Si+4HNO3→SiO2+4NO230 2H2OSiO2+6HF→H2SiF6+2H2O
Semiconductor wafer manufacturing in the past simply charged the etch tool with a batch of chemistry and etched wafers for a set period of time. The etch process was not tailored to individual wafers as no wafer specific information was available to the etch tool. Accordingly, the goal was to have the same etch profile on each wafer. When the chemistry (etchant) was fresh, the etch rate was at its highest rate. Each wafer etched consumed a portion of the active ingredients and accordingly, the etch rate continually diminished with use as shown in FIG. 1. When the chemical bath etch rate declined to the minimum acceptable level, the batch of chemistry was drained and new chemistry filled the tool, thereby restoring the etch rate. Chemistry usage was high due to the continuous replacement of the chemistry and wafers were not etched to the same depth so improvements were sought.
FIGS. 3A-3C illustrate the deficiencies described above and in particular, depict an example in which the etch recipe is calculated to remove a selected amount of material, in this case 10 microns, from each processed wafer using the same etch process (e.g., the same batch of chemistry, the same etch time, etc.).
For example, FIG. 3A depicts a first wafer 10 (W10) both pre and post etching. As shown, the first wafer 10 initially has a thickness of 300 microns and it is desired to remove 10 microns, thereby producing a wafer having a thickness of 290 microns. The first wafer 10, as shown, has a uniform profile defined by a planar top surface. The first wafer 10 is the first wafer to be exposed to the etchant. Since the chemistry is fresh and the first wafer 10 has a uniform profile, the etching process results in the desired 10 microns being removed from the first wafer 10. The post etch wafer thus has a uniform profile defined by a planar top surface.
FIG. 3B depicts a second wafer 20 (W20) both pre and post etching. As shown, the second wafer 20 does not have a uniform profile due to a center to edge variation (i.e., the edge is thicker). When the etchant used in the etch process to etch the first wafer 10 is used again and the same etch rate is intended to be maintained (i.e., removal of 10 microns), the resulting (observed) etch rate slightly diminishes (as indicated by the change in the etch depth) due to the chemistry losing some of its effectiveness. For example, the second wafer 20 has a thickness at the edge of 305 microns and at the center of 300 microns. Etching the second wafer 20 results in only 9.9 microns of material being removed as opposed to the 10 microns removed for the first wafer 10 due to not only the non-uniformity of the second wafer's profile but also due to the loss in effectiveness of the chemistry. This results in the post etch wafer still suffering from a lack on uniformity in that the top surface still has center to edge variation (the edge is thicker).
FIG. 3C depicts a third wafer 30 (W30) both pre and post etching. As shown, the third wafer 30 does not have a uniform profile due to a center to edge variation (i.e., the center is thicker). When the etchant used in the etch process to etch the first and second wafers 10, 20 is used again and the etch rate is maintained (i.e., intended removal of 10 microns), the resulting (observed) etch rate further diminishes (as indicated by the change in the etch depth). For example, prior to etching the third wafer 30 has a center thickness of 300 microns and an edge thickness of 293 microns. Etching the third wafer 30 at the same etch rate (i.e., intended to remove 10 microns) results in less than 10 microns being removed and in particular, in this example, only 9.8 microns of material is removed. This results in the post etch wafer still suffering from a lack on uniformity in that the top surface still has center to edge variation (the center is etched to 293.2 microns and the edge is etched to 283.2 microns).
It will thus be appreciated that the etching process described in the above Examples does not correct for the non-uniformity of the wafer in the wafer examples of FIGS. 3B and 3C.
Subsequent attempts were made to maintain etch rate by adding chemistry and this technique is referred to as spiking. Since the processes for each wafer are identical and etch times constant, the same volume of chemistry can be added to maintain etch rate as shown in FIG. 2. This method worked fairly well when the wafer geometries were large and the same process could be used on each wafer.
FIGS. 4A-4C illustrate the deficiencies described above and in particular, depict examples in which the etch recipe is calculated to remove a selected amount of material (e.g., 10 microns) from each processed wafer using the same etch process and same etch time. However, in this comparative example, chemistry was added (spiking) for each etch process after the first one. In the illustrated exemplary embodiment, a constant amount (e.g., 2.0 ml) of chemistry is added after an etch process is performed on a given wafer. In other words, a program with a 2.0 ml spike is implemented. It will be appreciated that the amount of chemistry to be added to refresh the etchant chemistry will vary and can be selected in view of the specific application. Thus, a 2.0 ml spike dosage that is added after performing each etching process is only exemplary in nature and is not limiting. The amount (volume) of the spike dosage is preferably an amount that is intended to refresh the chemistry (etchant) to allow the selected etch rate to be maintained for each subsequent etch.
For example, FIG. 4A depicts the first wafer 10 both pre and post etching. As shown, the first wafer 10 has a uniform profile and has a thickness of 300 microns and it is desired to remove 10 microns, thereby producing an etched wafer having a thickness of 290 microns. The first wafer 10 is the first wafer to be exposed to the etchant. Since the chemistry is fresh and the first wafer has a uniform profile, the etching process results in the desired 10 microns being removed from the first wafer 10. The post etch wafer thus maintains its uniform profile due to the uniform etch.
FIG. 4B depicts the second wafer 20 both pre and post etching. As shown, the second wafer 20 does not have a uniform profile due to a center to edge variation (i.e., the edge is thicker (e.g., edge thickness is 305 microns). When chemistry (i.e., 2.0 ml) is added to the batch of chemistry to maintain the etch rate, immediately after wafer 10 is etched and before wafer 20 is etched, the resulting (observed) etch rate is maintained and results in 10 microns of material being removed. However, since the second wafer 20 has a non-uniform profile (edge is thicker), the resulting etched wafer suffers from the same deficiencies as the pre etched wafer in that the resulting wafer continues to have center to edge variation (the edge is thicker) even though the chemistry was spiked. Thus, the edge is etched to 295 microns and the center is etched to 290 microns. Spiking the chemistry thus does not correct the non-uniform nature of the wafer, but does ensure the etch rate is constant and same amount of material is removed.
FIG. 4C depicts the third wafer 30 both pre and post etching. As shown, the third wafer 30 does not have a uniform profile due to a center to edge variation (i.e., the center is thicker). When chemistry (i.e., 2.0 ml) is added to the batch of chemistry to maintain the etch rate, the resulting (observed) etch rate is maintained and results in 10 microns of material being removed. However, the third wafer 30 has a non-uniform profile (center is thicker) and therefore, the resulting etched wafer suffers from the same deficiencies as the pre etched wafer in that the resulting wafer continues to have center to edge variation (the center is thicker) even though the chemistry was spiked. Thus, the edge is etched to 283 microns and the center is etched to 293 microns. Spiking the chemistry thus does not correct the non-uniform nature of the wafer.
Over time, geometries diminished and the demand for tighter uniformity between post etch wafers became a requirement. Subsequent etch tool developments to meet these new needs included on board measurement capability. Now instead of etching all wafers with the same process, an etch process could be tailored to a specific wafer. In the tailoring process, the non-uniformities from wafer grind could be taken into consideration. These non-uniformities include center to edge variations and incoming thickness variations. With the on board measurement capability, computer based algorithms to determine a customized etch profile, and the ability to change process parameters to deliver a unique etch profile to every wafer, a new breed of etch tool was able to deliver a value added process. In the above example, the wafers would not only be stress relieved but would now have TTV (total thickness variation) reduced and can be brought to a specific thickness post etch. These factors added value to the process; however, since each wafer received a unique process (different profile and different etch depth), the single value for chemistry spiking was no longer sufficiently accurate. In other words, uniform spiking of the chemistry does not optimize the wafer etching and can lead to both under-etching and over-etching.